1. Field of the Invention
This invention relates generally to the art of coating and lining fluid containers and conduits, and more particularly concerns systems and methods for chemically joining and sealing coatings and linings together for the purpose of coating and lining fluid containers and conduits.
2. Description of Related Art
Waste water and sewerage are frequently conducted through fluids conduits made of concrete. In some applications, the conduits are made of metal. Sewer systems frequently have reservoirs, containers and ponds, such as settling ponds which also are made of concrete and sometimes metal. Sewerage and waste water often contain material that is highly corrosive, especially with respect to concrete and metal. The closed systems that constitute most sewerage systems in use presently create anaerobic conditions ideal for the proliferation of anaerobic bacteria that result in hydrogen sulfide. Consequently, waste water pipes, sewage pipes, reservoirs, containers and ponds frequently are the subject of substantial corrosion and deterioration. The addition, corrosive material resulting from the digestive by-products of organisms existing within the waste water and sewer systems cause substantial deterioration of the concrete and metal conduits.
As used herein, the terms xe2x80x9cconduitsxe2x80x9d and xe2x80x9cpipes,xe2x80x9d are intended to include not only conduits and pipes, but also box tunnels and culverts, reservoirs, containers and ponds as discussed above. Such xe2x80x9cconduitsxe2x80x9d and xe2x80x9cpipes,xe2x80x9d as those terms are used herein, can include, for example, settling or processing xe2x80x9cpondsxe2x80x9d at sewage processing plants, back-up reservoirs and, in general, building structures involved with such processing.
The deterioration that occurs in such conduits and pipes frequently takes the form of physical decay of the walls of the conduits and pipes, so that the thickness of the walls is reduced. Significant amounts of the concrete or metal can be removed or corroded away over the years. In many instances, sewer and waste water conduits and like structures have walls so thin that the conduits have collapsed under the weight of traffic passing over them, and it has been reported that people have stuck their arms through them with relative ease.
Another undesirable effect of the corrosion and deterioration is that in the weakened condition that thin walls have, the walls crack more easily, resulting in leaks in the sewer system. Such leaks undermine the soil or prepared beds surrounding the concrete or metal pipes. Further, such leaks may contaminate the aquifer, pollute the waterways, and come to the surface to make the environment above unpleasant, as well.
Some of the more common corrosive materials within sewer conduits are sulfuric acid and hydrogen sulfide, which can turn into sulfuric acid under the conditions frequently found in sewer systems. Their corrosive effects may be readily appreciated. Sewer conduits normally have an anaerobic slime deposit along the wall of the conduit which is submerged below the liquid line or level. Various metal sulfates are among the more common ingredients of this anaerobic deposit. In the substantially liquid waste water, the sulfates reduce to sulfide ions, which combine with the hydrogen in the waste water to outgas above the liquid level as hydrogen sulfide.
Hydrogen sulfide, per se, can be highly corrosive to metals, of which some sewer pipes are made. Many concrete pipes have metal, as, for example, steel reinforcing ribs, beams and like support structures, which are sometimes called xe2x80x9crebar.xe2x80x9d
Oxygen from the air intakes in the sewer system, such as manholes, as well as oxygen from water condensing on the upper portions of the interior surfaces of the conduit wall which are not in contact with the liquid being conducted, interacts with bacteria within the conduit above the liquid level. The bacteria above the liquid level thus produces more hydrogen sulfide. Oxygen from the liquid and condensing from the water in the air reacts with the hydrogen sulfide gas to create highly corrosive sulfuric acid. The acid reacts with the calcium hydroxide in the cement of the concrete conduit, producing gypsum or calcium sulfate, which has been described as a soft corrosion product. The gypsum drops off the interior of the wall itself, making the wall thinner and substantially weakened. In this manner, whole chucks of the concrete wall are removed or vanish, reducing the thickness and strength of the wall.
Restoring such damaged and weakened conduits and pipes in the past has been accomplished primarily through two fundamental methods. In one basic modality, the concrete conduits are entered and fresh concrete is troweled or applied onto the walls to build the thickness back to the original dimension. This method is somewhat problematic in that applying concrete on the roof of the conduit requires special efforts and time. Frequently, such as in sewer conduits, for example, the repair must be performed within a specified time window. Usually, sewer conduits have a relatively lower flow rate or level during the hours between midnight and the normal waking hours of around seven in the morning. After the normal waking hours, most sewer conduits are so full that entry into the conduit and any repair are not possible.
A second modality involves a process of digging from the top surface down to the weakened conduit. The earth around the weakened conduit is excavated, and a layer of concrete of suitable thickness is poured around the weakened conduit to, in effect, provide a new conduit encasing the old, crumbling sewer pipe. This method has special problems, as well. For example, most such pipes are underneath roadways and streets. Excavating down to the sewer pipe requires removing the surface road or street and diverting traffic often for weeks at a time. The amount of concrete required to encase the old pipe is substantial. The manpower required for excavating, concrete pouring and road and street restoration is substantial, as well. Both in time and material, such repair or restoration is expensive, and the disruption to street traffic is costly to the public.
Other modalities exist, such as for example a method called xe2x80x9cslip lining,xe2x80x9d in which new pipe is inserted within the old, crumbling pipe to strengthen the pipe walls. Such methods reduce the diameter of the pipe or conduit and create problems with connecting lateral pipes joined to the pipe being repaired or restored.
Many of the methods used in the past result in a restored conduit which has a surface facing the effluent made of the same material that deteriorated before. This problem has been addressed by coating or lining the interior facing surfaces after restoration of the conduit, but such liners or coating have met with only varying levels of satisfaction.
In the past, many such coating have been made of material in which small pin holes develop, through which the corrosive substance seeps to corrode the concrete so coated xe2x80x9cbehindxe2x80x9d the lining or coating. The corrosion taking place behind the liner or coating not only deteriorates the concrete or metal conduit itself, and the lining or coating breaks loose from the conduit to add to the clogging within the conduit.
It has been known in the past to provide lining for the interior surfaces of such conduits and pipes. See, for example, U.S. Pat. No. 4,792,493 issued to Vernie L. Belcher and myself, and U.S. Pat. No. 5,268,392 issued to myself. Such linings greatly enhance the ability of the conduit to resist the corrosive effects of the many acids, caustics, toxins and organic material common in waste water and sewer systems.
In many situations, however, liners are applied to the interior of concrete pipe sections prior to inserting in the ground and connecting with other pipe sections to form the conduit. In one conventional technology, the liners are provided in sheets, and can be secured to the concrete by ribs having a xe2x80x9cTxe2x80x9d cross-section embedded in the concrete. Such sheets are normally in widths of approximately four feet. Multiple sheets are welded together to line specified lengths of concrete pipe, with adjacent sheets defining seams where the weld occurs.
The sheets sections, or as used herein simply xe2x80x9csheetsxe2x80x9d, require a joinder of serially adjacent sections at their joining edges. In the past, such joinder has been accomplished by a thin strip of PVC approximately one inch wide applied to cover the seam and over the two abutting edges from the adjoining sheets. Such strips are normally inserted at the rate of 180 lineal feet per man-day.
The seal of the joint between adjoining thermoplastic sheets inside sewer conduit, or even in a container is important because of the corrosive substances that likely will be contained or conducted within them. Should the seal fail and the corrosive substances seep behind the seal, through the space between the adjacent thermoplastic sheets to the grouting or other material, and even the concrete or metal behind the thermoplastic liners themselves, the underlying conduit or container is jeopardized.
Many such seals in the past have been accomplished by a heat seal method. Normal temperature in underground sewers usually range from 40 to 65 degrees Fahrenheit. The temperatures vary little over the seasons of the year because of the subterranean location of the conduits. In the heat seal method, the PVC strip is typically heated to some elevated temperature to fuse or heat seal the strip to the adjoining thermoplastic sheets, which themselves are PVC. Elevating the temperature of the PVC necessarily occurs when the sealing strip is heat sealed onto the PVC sheets. Raising the temperature as occurs in the heat sealing or fusing process can damage the PVC, i.e. dry out the PVC, causing the surface to crack and sometimes to shrink. Such sealing or fusing processes can tend to oxidize and decompose the PVC. Failure to raise the temperature of the materials sufficiently, however, can result in failure of the heat seal bond. In addition, frequently in the application of such heat seal welds, the installation process can be rushed, resulting in application of heat for an insufficient period of time to yield adequate bonding, resulting in subsequent bonding failures.
PVC contains plasticizers that can migrate in the heat sealing or fusing processes, changing some basic properties of the PVC. If the plasticizers migrate to the surface portions to which the sealing strip is to be joined, a joining surface having a higher concentrations of plasticizer will tend to make the surface not stick, or less capable of holding any surface joined to it.
Referring to FIGS. 1 to 3, illustrating a prior art seam sealing technique, a sewer conduit 10 is typically comprised of a cylindrical concrete pipe 12. Often, the concrete 12 is reinforced by steel rebar. A PVC sheet 14 lines the interior of the concrete pipe 12. The sheet 14 shown is of the xe2x80x9cTxe2x80x9d construction, having a series of parallel ribs 16 extending perpendicularly from a surface of the sheet 14, each rib ending in a perpendicular cross-rib to result in a xe2x80x9cTxe2x80x9d shaped cross-section. The sheet 14 is normally placed in position on the concrete form prior to pouring the concrete pipe into the form. When the pipe is removed from the form, the sheet 14 will be formed with the pipe 12. In some situations in repairing installed conduits, a liner 14 can be positioned within the pipe either as a form or supported by a form. Grout is then inserted behind the liner to fill the space between the liner and the concrete pipe to embed the xe2x80x9cTxe2x80x9d ribs.
The xe2x80x9cTxe2x80x9d shaped ribs 16 of the PVC sheet 14 are embedded into the concrete pipe 12. As shown, a first, single sheet 20 is defined by a width indicated by the dimension 22 and has a longitudinal dimension sufficient to extend around the interior circumference of the concrete pipe 12. As shown, the sheet 20 has an edge 24, shown in broken line in FIG. 1, to be joined to an adjacent edge 26, also shown in broken line FIG. 1, of a second sheet 28 having a width 30 and positioned seriatim next in the length of the concrete pipe 12 along the direction of the flow of the fluid, indicated by the flow arrow 32. In a conduit thousands of feet, or even several miles in length, many of such sheets 12 must be installed. Each of such sheets 20, 28 must be joined at their seams in such manner that prevents seepage between the joinder.
As may be better seen in FIGS. 2 and 3, a PVC strip 36 is positioned over and covers the adjoining edges 24, 26 to protect them from the substances flowing in the direction of the arrow 32 in the conduit 10. The PVC strip 36 usually is on the order of one inch in width. In the past, the strip has been heat fused onto the top surfaces of the abutting sheets 20, 28, covering the seam of the edges 24, 26. Hereinafter in this description, the term xe2x80x9ctopxe2x80x9d will be used to denote the surface facing the interior of the conduit 10; that is, facing the interior which conducts the fluid flowing 32 within the conduit or, in the case of the container, facing the fluid contained therein. Such a heat fusion required that a strong elevation of temperature be applied to the strip 36, which was sufficient to elevate the temperature of the covered sheets 20, 28 at least at the portions being covered by the strip 36. Further, the heat had to be applied for specified periods of time, depending on the ambient temperature and humidity conditions. Frequently, the seal obtained for such a prior art seam was insufficient, and the seal gave way, cracked, became separated, and the strip 36 became separated from its seam. The corrosive material contained in the conduit 10 seeped through the seam between the edges 24, 26 to attack the concrete pipe 12.
As is illustrated in FIG. 3, a common solution to the breakdown of any seal in the generally circular seams of the liner 14 is to provide a substantially wider additional strip 37 to cover the strip 36 and adjacent portions of the top surfaces of the abutting sheets 20, 28. However, application of such a wider sealing strip requires greater care and time to achieve satisfactory heat bonding, and it has been found that such a seal will eventually also deteriorate in the same manner as the original seal of the heat fused strip 36.
It is thus desired to have a method and a material for sealing thermoplastic sheets lining or coating a conduit, such as sheets made of PVC, that prevent corrosive substances conducted through the conduits from seeping behind the sheets used as liners. It is further desired to provide a method, and material that can be used to accomplish such seals without any elevation in temperature. It is desired to provide such methods and material that will accomplish a seam in a relatively short time and which will relatively simple to accomplish. The present invention meets these and other needs.
Briefly, and in general terms, the present invention provides for a method for joining thermoplastic sheets to each other at a seam defined by adjacent edges of the sheets, by a chemical weld that is accomplished within a short time, measured in minutes, and at ambient temperatures in sewer environments, that is, at temperatures as low as 40 degrees Fahrenheit or lower. A surface portion extending from the seam comprises polyurethane containing an excess isocyanate component. The seam between the thermoplastic sheets is covered by a strip of polyvinyl chloride having a corresponding facing surface portion that contains a resin with available hydroxyl groups for chemical covalent bonding with the surface portions of the thermoplastic sheets adjacent to the seam. The strip is smoothed to expel entrapped gas, if any. In another presently preferred embodiment, the seal can also be accomplished by preparation of a surface portion of a first thermoplastic sheet extending from the first thermoplastic sheet, and preparation of a surface portion of a second thermoplastic sheet extending from a corresponding mating edge of the second thermoplastic sheet, and then by overlapping the prepared mating surface portions of the first and second thermoplastic sheets, to define a seam by overlapping the edges of the two sheets. In this manner, the additional polyvinyl chloride strip can be eliminated. All of the sealing is accomplished at ambient temperature within a relatively short period of time.